This invention relates to removing hazardous contaminants such as radioactive radon gas (radon-222) and volatile synthetic organic chemicals (VOC's) from potable water supplies, particularly for residential use, and a process, apparatus and system therefor.
A. Radon Hazards
Radon is a colorless, odorless, radioactive gas produced from the natural decay of uranium. In outdoor air, radon is diluted and not recognized as a health hazard. However, when radon gas is trapped indoors in air or water, in sufficiently high concentrations, it can be dangerous. Radon has been shown in several epidemiological studies to be a very potent carcinogen that causes lung cancer in humans. In the 1986 publication "A Citizen's Guide to Radon", OPA-86-004, the United States Environmental Protection Agency (EPA) stated that scientists have estimated that about 5,000 to about 20,000 of the 130,000 lung cancer deaths in the United States in 1986 may have been caused by long term exposure to radon gas. This is considered to be the second leading cause of lung cancer, after cigarette smoking. Of these 5,000 to 20,000 deaths, about 500 to 1,500 have been attributed to radon from residential potable water supplies. The risk from water-borne radon is considered to be higher than the combined risk from all of the other man-made chemical contaminants in residential drinking water.
Since radon is an alpha emitter, the inhalation of radon gas is considered to be a much greater problem than the ingestion of water containing radon. If the radon is located within the digestive tract, the easily shielded alpha particles are likely to be absorbed by the fluids in the digestive tract, where the energy is harmlessly dissipated. On the other hand, there is no shielding material within the lungs to keep the alpha particles from impinging on the delicate lung tissue, if the radon is inhaled.
Uranium, and therefore radon, is frequently found in granite bedrock deposits, which are common throughout the United States and the world. It is estimated that there are approximately 10 million private bedrock wells in the United States, and that approximately 10% of them are contaminated with radon in excess of 1,000 pico-curies per liter (pc/l). Radon typically enters a home along with the water from a bedrock well. Because radon is a volatile gas, it is given off by the water during common household activities such as bathing, showering, and washing dishes. Thus, the radon given off by the water becomes an inhalation hazard. It has been estimated by the EPA that, on an average annual basis, each 10,000 pc/l concentration of radon in the water supply will translate into an indoor air concentration of 1 pc/l for homes having average air exchange rates. Of course, homes with a lower air exchange rate would be expected to have higher indoor radon concentrations.
The EPA is currently developing standards for acceptable levels of radon in public water supplies. The range of values for radioactivity concentration being presently considered run from 167 pc/l per to 40,000 pc/l (keynote address at "Radon Diagnostician Mitigation Workshop", Apr. 12, 1988, So. Me. Voc. Workshop, Portland, Me.). Although the EPA standard for public water supplies will not be directly applicable to private residences, it is expected that the public will accept and regard this standard as the safe level.
Naturally occurring radon in water generally emanates from the radium (a decay product of uranium) in bedrock surrounding an underground well and through which the water going to the well flows. This is in sharp contrast to the most commonly known sources of contamination in water, which generally arise from remote point contamination sources such as leaky pipes and tanks or improperly disposed of waste materials. As such, some known methods of purifying water of organic contamination are not clearly applicable to removing radon from water.
There are fundamentally two known methods for treatment of water supplies for radon removal in the prior art: decay storage and spray aeration. Conceptually, the simplest example of decay storage is a large baffled water storage tank. Since radon has a radiological half life of only 3.785 days, simply holding the contaminated water in a storage tank for approximately a month will greatly reduce the radon level (by approximately 991/2%). One problem with this type of decay storage is that it requires a large tank which maintains essentially plug flow conditions to prevent backmixing.
Another example of decay storage requires accumulation of the radon on an adsorbent material such as activated carbon. Since the radon decays relatively rapidly, the concentration of radon on the adsorbent bed will initially increase, and then gradually reach an equilibrium with the influent radon concentration in the water. One disadvantage with this type of system is that the adsorbent bed gives off radioactivity, primarily in the form of gamma rays, as the radon decays, which may itself present a health hazard. A second disadvantage is that it is difficult, at least at the present time and the foreseeable future, to legally dispose of the radioactive carbon filter when it becomes fouled by other water-borne contaminants such as iron, sediment or bacteria.
For spray aeration systems, such as is disclosed in U.S. Pat. No. 4,371,383 to Rost, radon removal efficiencies are reported to be between 80% and 90% for three cycles of spraying (approximately 50% on each separate spray cycle). Therefore, in order to achieve an overall removal efficiency of 99%, the water must be recycled through the spray aeration device 7 to 8 times. This is disadvantageous in that it would require a spray tank that is relatively large (on the order of 100-150 gallons), to provide a sufficient quantity of treated water for use in the home.
A third method is the subject of Applicant's co-pending patent application, U.S. Ser. No. 072,553 for a Radon Removal System. That system includes a packed mass transfer material aeration column which is sized to fit within a single story of a residential building. The contaminated water splashes down through the packing material, while air blown up through the column packing comes into contact with the radon-laden water and evaporates the radon out of the water.
B. Volatile Synthetic Organic Chemical Hazards
In recent years, the contamination of drinking water by organic contaminants (known as volatile synthetic organic chemicals or VOC's) has become recognized as a serious problem in the United States and elsewhere. As stated by the United States Environmental Protection Agency (EPA) in "National Primary Drinking Water Regulations; Volatile Synthetic Organic Chemicals; Final Rule and Proposed Rule", 50 Fed. Reg. 46880-933 (Nov. 13, 1985), exposure to high levels of certain VOC's has been shown to result in a variety of acute and toxic effects in animals, including damage to liver and kidneys, as well as central nervous system effects and cardiovascular changes. Carcinogenic effects have also been demonstrated from exposure to certain VOC's, at least in studies using animals. The EPA has concluded that six VOC's in particular, (benzene, vinyl chloride, carbon tetrachloride, 1,2-dichloroethane, trichloroethylene, and tetrachloroethylene), were either known or probable carcinogens in humans (50 Fed. Reg. at 46886).
From 1975-1982, the EPA conducted six national surveys of the concentrations of certain contaminants in public drinking water systems. The EPA and the United States National Academy of Sciences (NAS) have used various analytic models to also estimate the cancer risk for these drinking water contaminants. The following table, derived from 50 Fed. Reg. 46882-83, presents a comparison of the range of concentrations found by the EPA in the six national surveys to the range of concentrations estimated to produce a 10.sup.-5 increased cancer risk, for each of the six known or probable VOC carcinogens:
______________________________________ Concentration Concentration For 10.sup.-5 Cancer Compound Found (.mu.g/l) Risk (.mu.g/l) ______________________________________ Benzene 0.2-22 .about.13 Vinyl chloride 0.2-66 0.15-10 Carbon tetrachloride 0.5-30 2.7-45 1,2-dichloroethane 0.2-21 3.8-7 Trichloroethylene 0.2-160 26-45 Tetrachloroethylene 0.1-69 6.7-35 ______________________________________
Thus it is clear that the presence of VOC's in drinking water constitutes a significant public health hazard.
The class of contaminants that are known as volatile synthetic organic chemicals (VOC's) typically includes, in addition to the six compounds discussed above, the following: